Phillip Resnick took the stand in Manhattan Supreme Court and said Yoselyn Ortega suffered from hallucinations and delusions. (Jefferson Siegel/New York Daily News)

A renowned forensic psychiatrist who diagnosed the Unabomber's mental illness, testified Thursday that a Manhattan nanny accused of murdering two children didn't know what she was doing.

Phillip Resnick took the stand in Manhattan Supreme Court and said Yoselyn Ortega suffered from hallucinations and delusions and had a "dissociative episode" when she allegedly killed Lulu and Leo Krim, 6 and 2, in their Upper West Side home on Oct. 25, 2012.

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"Ortega was psychotic. I mean she was out of touch with reality," Resnick said.

The expert, who gave his opinion in the case of prodigy-turned-terrorist Ted Kaczynski, rejected prosecutors' claims that Ortega, 55, murdered the kids to spite their mother.

Yoselyn Ortega allegedly killed Lulu and Leo Krim, 6 and 2, in their Upper West Side home on Oct. 25, 2012. (Lulu and Leo Fund/AP)

Resnick, whose resume includes work on the Scott Peterson and Jeffrey Dahmer cases, said he didn't buy that.

The people in Ortega's circle "talked about how much she loved the children so it just to me makes no sense at all how a woman, even if she were angry with Mrs. Krim, would kill two children she loved," he said.

"She could have attacked Mrs. Krim. She could have killed Mrs. Krim in anger. But to kill two children she loved out of revenge simply does not add up or make sense."

If the jury believes Yoselyn Ortega’s insanity defense, she'll likely spend the rest of her life in a psychiatric institution. If she's convicted of murder she faces life in prison. (Jefferson Siegel/New York Daily News)

Resnick will be on cross-examination Friday.

He is the second defense doctor to give the opinion that Ortega should not be held legally responsible for the murders.

If the jury believes Ortega's insanity defense, she'll likely spend the rest of her life in a psychiatric institution. If she's convicted of murder she faces life in prison.